Start testing against Python 3.9 (#14303)

Author: brettcannon

.github/workflows/insiders.yml

@@ -6,7 +6,7 @@ on:
       - main
 
 env:
-  PYTHON_VERSION: 3.8
+  PYTHON_VERSION: 3.9
   MOCHA_REPORTER_JUNIT: true # Use the mocha-multi-reporters and send output to both console (spec) and JUnit (mocha-junit-reporter). Also enables a reporter which exits the process running the tests if it haven't already.
   CACHE_NPM_DEPS: cache-npm
   CACHE_OUT_DIRECTORY: cache-out-directory
@@ -142,7 +142,7 @@ jobs:
         # macOS runners are expensive, and we assume that Ubuntu is enough to cover the UNIX case.
         os: [ubuntu-latest, windows-latest]
         # Run the tests on the oldest and most recent versions of Python.
-        python: [2.7, 3.8]
+        python: [2.7, 3.9]
         test-suite: [ts-unit, python-unit, venv, single-workspace, multi-workspace, debugger, functional]
     steps:
       - name: Checkout
@@ -372,7 +372,7 @@ jobs:
         # We're not running CI on macOS for now because it's one less matrix entry to lower the number of runners used,
         # macOS runners are expensive, and we assume that Ubuntu is enough to cover the UNIX case.
         os: [ubuntu-latest, windows-latest]
-        python: [3.8]
+        python: [3.9]
     steps:
       # Need the source to have the tests available.
       - name: Checkout